1. Field of the Invention
The present invention relates to a four-wheel steering apparatus. More particularly, the present invention relates to a four-wheel steering apparatus utilized as a steering apparatus for an automobile, such that when the automobile changes lanes, not only the direction of the front wheels changes, but the direction of the rear wheels is also allowed to change. Accordingly the turning radius becomes smaller and the traveling stability of the vehicle is maintained.
2. Related Background Art
In order to change lanes with ease in a narrow road, the turning radius of an automobile should be small. Also, the stability of the vehicle should be maintained in changing lanes at high-speed traveling. To this end, there has been in use in recent years a four-wheel steering apparatus wherein not only the front wheels are moved but the rear wheels are also moved when the steering wheel is operated.
The steering center of each of the rear wheels is positioned backward from the point of the force applied (grounding point), as considered in the advancing direction of the vehicle, and the rear-wheel output shaft, which provides steering angles for the aforesaid rear wheels, is supported resiliently with a slight freedom of displacement in the axial direction. Hence, the stability of the vehicle is maintained in changing lanes at high-speed traveling by providing the rear wheels with the steering angle of the same phase as the front wheels on the basis of centrifugal force. This method has been proposed conventionally as disclosed in Japanese Utility Model Publication No. 1-41656, for example, and is in use practically in some quarters.
However, in the case of the conventional structure disclosed in the above-mentioned publication, there occurs a case where an optimum steering angle is not necessarily given to the rear wheels depending on the vehicle velocity because the structure is such that by mechanically coupling the front-wheel steering mechanism and the rear-wheel steering mechanism, a steering angle is given to the rear wheels in a predetermined relationship irrespective of the vehicle velocity when a steering angle provided for the front wheel exceeds a predetermined degree.
It is possible to provide the rear wheels with a steering angle under an optimum condition at all times if the front-wheel steering mechanism and the rear-wheel steering mechanism are coupled electrically, so that the steering angle given to the rear wheels may be varied not only by the steering angle given to the front wheels but also by the vehicle velocity. However, if the front-wheel steering mechanism and the rear-wheel steering mechanism are simply coupled electrically, it is necessary to enhance the so-called fail-safe mechanism substantially which leads to the unavoidable increase in its manufacturing cost. More particularly, when coupling the front-wheel steering mechanism and the rear-wheel steering mechanism, if the steering angle given to the rear wheels at a high-speed traveling should also be controlled by electrically, a double- or triple-fail-safe mechanism should be incorporated to prevent the vehicle from being in a critical condition even when a controller should become out of order at high-speed traveling, thus making the manufacturing cost significantly high.